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December 12, 2008

MESSENGER Results To Be Presented At AGU Fall Meeting

by Sam Savage

Members of the MESSENGER science team will present a range of new findings from the spacecraft's studies of the planet Mercury during the American Geophysical Union (AGU) Fall Meeting next week in San Francisco.

"The principal goal of the MESSENGER mission is to gather global observations of Mercury from orbit about the planet, beginning in 2011, but our first two flybys produced a wonderful abundance of new information about the least known of our sister planets," says MESSENGER Principal Investigator Sean Solomon of the Carnegie Institution of Washington. "The AGU Fall Meeting is the first opportunity for most science team members to share results from the second Mercury flyby with our scientific community colleagues."

MESSENGER scientists will present 44 papers "“ some in a webcast session "“ that span the broad diversity of planetary phenomena observed by MESSENGER's instruments when the probe flew past Mercury in January and October 2008. Some of these papers cover discoveries regarding the environment around Mercury, such as how solar-derived particles originate and blast off into space. "Flying that close to the Sun, MESSENGER gave us our best look yet at several solar phenomena," says MESSENGER Project Scientist Ralph McNutt, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "We're detecting dust and other particles near the Sun and gaining insight on neutron behavior in this dynamic environment. Not only are we learning more about how these materials speed toward Earth and beyond, but we're learning about the conditions that MESSENGER will encounter when it becomes the first spacecraft to orbit the innermost planet."

William Feldman of the Planetary Science Institute will present new analyses that add to our knowledge of the dynamics of solar flares. Nuclear interactions in solar flares can produce energetic neutrons, and these neutrons are important keys to understanding flare processes. However, free neutrons have lifetimes of only about 15 minutes and decay into protons, electrons, and antineutrinos. Only the fastest (highest-energy) neutrons from solar flares can therefore reach Earth before they decay.

MESSENGER, at about two times closer to the Sun than Earth, made the first observations of lower-energy (less than 10 million electron volts) solar neutrons from a modest-sized solar flare on December 31, 2007.The neutrons from this New Year's Eve flare continued to be observed for many hours, unlike the optical emissions from the flare. The duration of neutron production implies that the acceleration and storage of atomic particles in the solar corona continued for a long time. "The fact that this time is considerably longer than the mean lifetime of a neutron indicates that neutrons at the Sun must have been continuously produced," Feldman explains. "The extended production of neutrons means that a large region near the Sun will be populated by neutron-decay protons, which can seed the later acceleration of protons by coronal mass ejection-driven shock waves that can produce a dangerous radiation environment for all satellites in interplanetary and planetary space."

An extended source of heavy ions and molecules was discovered using data from MESSENGER's novel Fast Imaging Plasma Spectrometer by taking ion composition measurements prior to and following MESSENGER's two Mercury flybys. The University of Michigan's George Gloeckler will report on the interactions of the solar wind with a distributed "inner source" of dust and other material orbiting the Sun. "The existence of an inner source was inferred from earlier observations of energetic singly-charged carbon with the Solar Wind Ion Composition Spectrometer on Ulysses at much larger distances from the Sun," Gloeckler says. "Being closer to this inner source with MESSENGER, we now find not only carbon and water-group ions but also many other elements and molecules with masses as high as 130 that of a proton, including Na, Mg, K, Ca, and Fe compounds." Gloeckler speculates that this inner source of material includes debris from Sun-grazing comets, dust particles, and possibly larger objects orbiting the Sun.

MESSENGER's Mercury flyby observations also revealed noteworthy differences in the planet's magnetosphere between January and October and "“ coupled with ground-based observations "“ have led to new discoveries about the planet's exosphere. James Slavin of NASA's Goddard Space Flight Center will present MESSENGER measurements indicating that a form of strong interaction between interplanetary and planetary magnetic fields, termed "reconnection," is approximately three times as effective in channeling solar wind energy into Mercury's near-space environment as it is at Earth.

Reconnection is known to control the rate of energy transfer from the solar wind to planetary magnetospheres. "The intensity of this energy input greatly influences the frequency and duration of the magnetic storms that accelerate charged particles, which modify planetary surfaces, produce aurorae in planetary atmospheres, and can affect spacecraft," Slavin says. "This MESSENGER result supports the theories that predict such an increase in the rate of reconnection as a result of the increase in the strength of the interplanetary magnetic field at Mercury's distance from the Sun. The high rate of energy input into Mercury's magnetic field in the MESSENGER measurements means that magnetic storms, which have durations of weeks at Jupiter and Saturn and an hour at the Earth, are expected to be much more frequent at Mercury but to last for only a few minutes."

During MESSENGER's first Mercury flyby in January, and again three weeks after the second flyby in October, ground-based observations of emission from the planet's sodium exosphere were obtained at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona, concurrently with observations from the probe's Ultraviolet and Visible Spectrometer (UVVS) on the Mercury Atmospheric and Surface Composition Spectrometer instrument.

The sodium exosphere has been observed via ground-based observations to be highly asymmetric and variable, at times to be peaked near the sub-solar point and at other times peaked at one or both poles. MESSENGER's UVVS observations of the sodium tail revealed a north-south asymmetry near the planet during the first flyby. The cause of the high-latitude enhancements in the sodium exosphere has been variously ascribed to solar wind ion impacts, to radiation pressure, to inherent surface compositional and physical differences, and to cold trapping. Over the course of the mission, the science team hopes to determine the partitioning of sodium and other species between the thermal and non-thermal components to determine the processes of desorption from surface materials.

Rosemary Killen, of the University of Maryland, will discuss new discoveries derived from observations of sodium, calcium, and magnesium in the nightside and tail regions of the exosphere. "The dawn/dusk asymmetries are particularly intriguing," Killen says. "While sodium is a volatile element, calcium is not easily vaporized. Magnesium can be found in both volatile and refractory minerals. So a comparison of their relative distributions and time variations in the exosphere will help determine the important processes involved in loss and transport of surface materials."

The MESSENGER AGU presentations will be made in three separate sessions on Monday and Tuesday, December 15-16. The first of those sessions will be webcast. To view the session, go to the AGU Fall Meeting web page at http://www.agu.org/meetings/fm08/ and click on the appropriate session at the scheduled time (Pacific time). Individual speakers can be found in the Fall Meeting Scientific Program web pages.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Sean Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. APL built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

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Image Caption: MESSENGER at Mercury - Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury. MESSENGER launched from Cape Canaveral Air Force Station, Fla., on Aug. 3, 2004, and will begin a yearlong orbital study of Mercury in March 2011. Though the Sun is up to 11 times brighter at Mercury than we see on Earth and surface temperatures can reach 450 degrees Celsius (about 840 degrees Fahrenheit), MESSENGER's instruments will operate at room temperature behind a sunshade of heat-resistant ceramic fabric. The spacecraft will also pass only briefly over the hottest parts of the surface, limiting exposure to heat reradiated from the planet. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington